A variable cycle gas turbine engine, such as disclosed in U.S. Pat. No. 4,175,384 to Wagenknecht, et al., and incorporated herein by reference is capable of acting as both a turbojet and a turbofan by varying the bypass ratio of the engine. A conventional variable cycle gas turbine engine will include a fan, a midfan or low pressure compressor, a high pressure compressor, combustor, high pressure turbine, low pressure turbine, and an augmentor in serial flow relationship. A bypass duct supplies bypass air from the fan and the low pressure compressor to the rear VABI where the bypass air can be returned to the core engine gas flow. Conventionally, the rear VABI is located aft of the low pressure turbine and forward of the augmentor. An augmentor conventionally includes a diffuser section, a flameholder and an augmentor screech suppressor. All or a portion of the bypass flow can be diverted to an augmentor cooling plenum where it can be injected through the augmentor cooling liner which includes a screech suppressor liner. This coolant flow protects the augmentor liner, as disclosed in U.S. Pat. No. 4,183,229 to Simmons. In order for bypass air to flow through the augmentor liner into the augmentor, it is necessary for the pressure of the bypass air in the cooling plenum to be higher than that of the core airflow through the augmentor. It has also been found that in order to enhance the performance of a variable cycle engine and avoid fan stall or compressor stall, it is necessary that the appropriate pressure ratio between those areas of the engine and the pressure in the augmentor be controlled. For additional information concerning the necessity to maintain the appropriate pressure ratios in a variable cycle engine and the use oaf a rear VABI for such control, reference can be had to co-pending application Ser. No. 07/504,380.
It has been found that in order to enhance the turbojet cycle of a variable cycle engine, the rear VABI needs to provide a better seal when in a closed position than has been previously achieved in order to conserve bypass air for coolant flow through the augmentor liner. This is particularly difficult considering manufacturing and assembly tolerance stackup that cause differences in how much travel is necessary for a VABI door or valve mechanism to close at different circumferential locations. It has also been found that to enhance the turbofan cycle of a variable cycle engine, the rear VABI should be configured to preserve momentum of the bypass airflow as it is injected into the core airstream. Preserving the momentum of the bypass airflow is best achieved when the velocity of the bypass airflow approximates the velocity of the core gas flow. It has been found that this velocity matching is usually closest aft of the diffuser. It has also been found that prior VABI configurations injecting relatively cool air forward of the augmentor can create unstable flame and auto ignition problems in the augmentor. Further, there have been problems with hot and cold streaks as cool bypass air is injected into the hot core gas airstream through prior VABI configurations, inducing thermal stresses due to the differing temperature gases flowing past flowpath surfaces aft of the VABI. Attempting to locate the rear VABI aft of the flameholders and diffuser lengthens the distance between the flameholder and screech suppressor, dimishing the suppressor's effectiveness. This also introduces new problems in configuring the VABI to fit within the smaller bypass duct area available aft of the diffuser.